During the drilling and completion of oil and gas wells, various wellbore treating fluids are used for a number of purposes. For example, high viscosity gels are used to create fractures in oil and gas bearing formations to increase production. High viscosity and high density gels are also used to maintain positive hydrostatic pressure in the well while limiting flow of well fluids into earth formations during installation of completion equipment. High viscosity fluids are used to flow sand into wells during gravel packing operations. The high is viscosity fluids are normally produced by mixing dry powder and/or granular materials and agents with water at the well site as they are needed for the particular treatment systems for metering and mixing the various materials.
In order to prevent formation fluids from entering the wellbore, the hydrostatic pressure of the drilling fluid column in the wellbore should be greater than the pressure of the formation fluids. The hydrostatic pressure of the drilling fluid column is a function of the density of the drilling fluid and depth of the wellbore. Accordingly, density is an important property of the drilling fluid for preventing the undesirable flow of formation fluids into the wellbore. To provide increased density, weighting agents are commonly included in drilling fluids. Weighting agents are typically high-specific gravity, finely ground solid materials. As referred to herein, the term “high-specific gravity” refers to a material having a specific gravity of greater than about 2.6. Examples of suitable weighting agents include, but are not limited to, barite, hematite, ilmentite, manganese tetraoxide, galena, and calcium carbonate.
As wellbores are being drilled deeper, the pressure of the formation fluids increases. To counteract this pressure increase and prevent the undesired inflow of formation fluids, a higher concentration of weighting agent may be included in the drilling fluid. However, increasing the concentration of weighting agent may be problematic. For example, as the concentration of the weighting agent increases, problems with particle sedimentation may occur (often referred to as “sag”). Among other things, particle sedimentation may result in stuck pipe or a plugged annulus. Particle sedimentation may be particularly problematic in directional drilling techniques, such as horizontal drilling. In addition to particle sedimentation, increasing the concentration of the weighting agent also may increase the viscosity of the drilling fluid, for instance. While viscosification of the drilling fluid may be desired to suspend drill cuttings and weighting agents therein, excessive viscosity may have adverse effects on equivalent circulating density. For example, an increase in the equivalent circulating density may result in an excessive increase in pumping requirements for circulation of the drilling fluid in the well bore.
Several techniques have been utilized to prevent undesired particle sedimentation while providing a drilling fluid with desirable rheological properties. For instance, decreasing the particle size of the weighting agent should create finer particles, reducing the tendency of the particles to settle (SAG). However, the increased number of particles of a reduced particle size may also cause an excessive increase in viscosity. One approach to reducing particle size while maintaining desirable rheology involves utilizing particles of a reduced size while limiting the number of particles that are very fine (below about 1 micron).
The powder or granular treating material is normally transported to a well site in a commercial or common carrier tank truck. Once the tank truck and mixing system are at the well site, the dry powder material must be transferred or conveyed from the tank truck into a supply tank for metering into a mixer as needed. The dry powder materials are usually transferred from the tank truck pneumatically. The dry powder may also be transported in bags, which are loaded into a hopper and then transferred to the mixing apparatus.
Many problems complicate the transference of fines (particles with an effective diameter less than about 6 μm). Typically, as fines are stored, they have a natural tendency to self-compact. Compaction occurs when the weight of an overlying substance results in the reduction of porosity by forcing the grains of the substance closer together, thus expelling fluids (e.g., water), from the pore spaces. However, when multiple substance fines are intermixed, compaction may occur when a more ductile fine deforms around a less ductile fine, thereby reducing porosity and resulting in compaction.
Pneumatic transport of micronized (less than 10 micron particle size) barite powder using conventional means also suffers complications due to the resulting introduction of moisture and the tendency of micronized barite to adhere to surfaces.